Avoid creating customer contacts from chat

Content

In my case Customers and Non Customers can access to the chat service, but i don't want that if one Customer access for first time (or qith another email that i don't have in the database) generates a new contact row, i want to force agents to associate this new email to an existing contact row.

Does anyone knows if there's a configuration to avoid auto generate new contact row?

Ps. I Load all my new customers with all the info with an automated process.

Thanks 

Regards

Version

Service Cloud